it seems an expensive activity for little benefit.

What have you got against taking the bag with you for a week?

It will also mean that you have to allow time for check in in Sydney before the flight to Auckland. If you are a bit delayed on arrival into Sydney and have to collect the bags from storage and get them checked in before check in closes for AKL , you are increasing the risk of being too late to check the bags in

I would take the bags and save the $100 unless there is a v good reason not to take them